1. Field of the invention
The present invention relates to an apparatus for measuring respiratory gas flow by utilizing dynamic pressure, and more particularly to an apparatus for measuring respiratory gas flow, which includes a disposable respiratory tube assembly assembled with a measurement module, so as to not only improve the accuracy in the measurement of the respiratory gas flow but also reduce the manufacturing cost of the apparatus.
2. Description of the Prior Art
In an examination of vital capacity, which is a clinical examination or clinical practice, change in the capacity of a human lung during respiration is continuously recorded and analyzed, so as to estimate and present all intermediary parameters, thereby assisting in diagnosing respiratory impairments.
As an apparatus for measuring respiratory gas flow which has been most widely used up to now, generally known to the public, are apparatuses of pneumotach type, in which a resistive element is disposed in the middle of a respiratory tube, and pressure difference between both sides of the resistive element is measured, thereby evaluating a flow quantity of the respiratory gas.
In order to measure the respiratory gas flow by means of such an apparatus of pneumotach type, employed is a relational expression between the flow quantity and the static pressure, which is the Rohrer""s equation, a quadratic function expressed as the following equation 1.
xcex94P=R0*F+R1*F2xe2x80x83xe2x80x83Equation 1
In equation 1, xcex94P is a pressure difference, R0 and R1 are constants, and F is a flow quantity of respiratory gas.
As apparent from equation 1, when the apparatus of pneumotach type is utilized in measuring the respiratory gas flow, two constants R0 and R1 should be determined, thereby not only complicating the measuring process but also inevitably increasing errors of the measurement at least by two times in comparison with measurement of the respiratory gas flow, which employs a single constant. That is, the measurement of the respiratory gas flow by means of the pneumotach type apparatus could have bad linearity and incorrect null adjustment, thereby deteriorating the measurement accuracy.
In order to overcome such problems as described above, U.S. Pat. No. 5,060,655, issued to Rudolph, discloses a pneumotach apparatus for measuring gas flows, which employs a respiratory tube having resistive elements to the respiratory gas flow in the respiratory tube, so that R1 in equation 1 is zero.
However, in the Rudolph""s pneumotach apparatus, the resistive elements have very complicated construction and their manufacturing cost is expensive, so that it is difficult to manufacture and sell a respiratory tube having the resistive elements as a disposable apparatus. Therefore, the respiratory tube must be used permanently or repeatedly, thereby causing severe sanitary problems.
Further, in such apparatuses of pneumotach type, the resistive elements are located in the middle of the path of an examined person""s respiratory gas, so as to disturb the examined person""s respiration and thereby change a flow signal which represents the examined person""s respiratory performance, thereby deteriorating the reliability of the examination for the respiratory performance.
In the meantime, U.S. Pat. No. 5,038,773, issued to Norlien, et al., discloses a respiratory gas flow measuring system, which measures the respiratory gas flow by measuring dynamic pressure instead of the static pressure, so as to overcome the above-mentioned problems.
FIG. 7 is a left side view of a cylindrical respiratory tube or a tubular barrel, which is employed in the Norlien""s respiratory gas flow measuring system.
As shown in FIG. 7, the conventional respiratory tube 1 contains a pair of ribs 2, which intersect at their midpoints to form a cross and centrally disposed relative to a midpoint of the respiratory tube 1. Fine apertures 4 for sampling the flow of the respiratory gas are formed near distal ends of the ribs 2 and at the center of the respiratory tube 1 at which the ribs intersect. The fine apertures 4 are symmetrically arranged, so as to the respiratory gas flow in both directions.
When the respiratory gas flow is measured by the respiratory gas flow measuring system as described above, the following relational expression, equation 2, is established between the respiratory gas flow and the dynamic pressure of the respiratory gas, provided that the respiratory gas flow has a constant velocity in the radial direction of the cylindrical respiratory tube.                     F        =                              A            *            u                    =                                                    A                *                                                      2                    ρ                                                  *                                                      P                    D                                                              ∝                                                P                  D                                                      =                          S              *                                                P                  D                                                                                        Equation        ⁢                  xe2x80x83                ⁢        2            
In equation 2, A is a sectional area of the cylindrical respiratory tube, S is a proportional factor, PD is the dynamic pressure, u is velocity of the respiratory gas, and xcfx81 is density of the respiratory gas.
Equation 2 shows that the quantity of the respiratory gas flow is proportional in principle to the square root of the dynamic pressure and there exits only one proportional factor S in equation 2. Therefore, it can be understood that the measurement errors are reduced and the null adjustment is more simplified in the respiratory gas flow measuring system, in comparison with the apparatus of pneumotach type.
The Norlien""s respiratory gas flow measuring system as described above has considerably overcome the existing problems such as the disturbance of respiration by the resistive elements and the complicated construction of the resistive elements.
However, in the Norlien""s respiratory gas flow measuring system as described above, the respiratory gas flow is sampled only at four location near a cylindrical wall of and at the center of the tubular barrel, but is not sampled between the center and the cylindrical wall of the tubular barrel, so that the measurement error of the respiratory gas flow is inevitably large.
Further, in the Norlien""s respiratory gas flow measuring system, the respiratory tube, or the tubular barrel, is made from plastic and has a complicated construction, thereby increasing the manufacturing cost, so that the Norlien""s respiratory gas flow measuring system has a low economical efficiency when it is used as a disposable apparatus.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide an apparatus for measuring respiratory gas flow, which provides a high accuracy in measurement of the respiratory gas flow and reduces the manufacturing cost of the apparatus, in comparison with the conventional disposable apparatus for measuring respiratory gas.
In order to accomplish this object, there is provided an apparatus for measuring respiratory gas flow, the apparatus comprising: a measurement module containing a differential pressure sensor and a thermistor; and a respiratory tube assembly detachably assembled with the measurement module, wherein the respiratory tube comprises: a cylindrical tube body having two rod holes formed through a cylindrical wall of the cylindrical tube body, the rod holes being disposed at opposite ends of a diameter of the cylindrical tube body, each of the rod holes having a shape of a diamond; and a sensing rod detachably inserted through the rod holes, the sensing rod having an appearance of a number xe2x80x9c1xe2x80x9d, the sensing rod having a cross-section shaped like a diamond, the sensing rod having a plurality of sampling pores and two air passages formed vertically through the sensing rod, the air passages being separated from each other, the sampling pores being formed through both sides of a sensing rod body of the sensing rod, first half of the sampling pores facing toward an entrance of the respiratory tube assembly and communicating with a first one of the air passages, second half of the sampling pores facing toward an exit of the respiratory tube assembly and communicating with a second one of the air passages, the first half of the sampling pores being located symmetrically to the second half of the sampling pores, wherein locations of the sampling pores are determined by an equal area division method in which a semi-circular section of the cylindrical tube body is divided into a plurality of annuluses with equal areas and the sampling pores are formed at the positions also equally dividing the areas of the annuluses which the pores belong to.
It is preferred that the sensing rod comprises the sensing rod body, a pyramidal head formed on the sensing rod body, and a base formed under the sensing rod body, which are integrated with each other, the pyramidal head having a lower surface whose area is larger than a sectional area of the sensing rod body, so as to form an upper jaw between the sensing rod body and the pyramidal head, the base having a sectional area larger than the sectional area of the sensing rod body, so as to form a lower jaw between the base and the sensing rod body, the upper jaw and the lower jaw enabling the sensing rod to be stably fixed in the respiratory tube assembly.